Meet America's 'Goon Gun': The M2 4.2-Inch Mortar That Won World War II

February 14, 2020 Topic: History Blog Brand: The Buzz Tags: World War IIMortars

Meet America's 'Goon Gun': The M2 4.2-Inch Mortar That Won World War II

With a good rate of fire and a hefty punch, the 4.2-inch chemical mortar was an unsung weapon of the U.S. Army in World War II.

Key point: While the mortar units were a valuable asset, they were not without their problems.

In the last days of March 1945, a soldier named Carl Getzel sat on a hill outside the city of Aschaffenburg and watched as it was slowly destroyed. The young private was part of the American 157th Infantry Regiment, 45th Division. His unit was tasked to take this town but it had been hard going; the German troops inside the city were putting up a determined defense.

At echelons far above Carl, the decision was made to bombard the city. Artillery and air strikes pounded the once-beautiful town, but from his vantage he saw the a third, little known weapon wreak its own havoc. Not far behind him sat Company C, 2nd Chemical Mortar Battalion, equipped with the M2 4.2-inch mortar. Bombs from the mortars flew overhead to land in the city. As he watched, the projectiles crashed into buildings, bringing several down. It was an awesome display of firepower; Company C would fire 1,519 rounds of ammunition into Aschaffenburg during the last three days of March alone.
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The Making of the Chemical Mortar

Although this account describes the 4.2-inch mortar (commonly nicknamed the “four-deuce”) as a close support weapon for frontline troops, the weapon was originally conceived for delivering poison gas and smoke munitions. Development of the M2 began after World War I as an attempt to improve the British-designed Stokes-Brandt 4-inch mortar. Since the intent was to utilize the new mortar solely for chemical weapons (smoke being considered a chemical weapon of sorts), the weapon came under the control of the U.S. Army Chemical Warfare Service. That branch wanted a weapon that had a longer range and was more mobile than the Stokes design. In addition, it wanted a weapon that had the normal advantages of a mortar: high rate of fire, the ability to deliver firepower quickly and to go into action in a short time.

Mortars are smaller and lighter than cannon of comparable caliber and fire at a high angle, their rounds arcing high into the air and coming down at a nearly vertical angle to the ground. This enables mortars to fire at targets in defilade, hidden behind objects such as buildings or hills that would mask artillery fire. Mortar ammunition, technically called “bombs,” is not subjected to the same pressures as artillery shells so it can be made with thinner walls and of cheaper metals, enabling it to carry higher payloads of explosives or chemicals. The primary disadvantage of a mortar is its relatively short range.

Initial experiments conducted after World War I used leftover Stokes weapons and munitions, but these trials resulted in the improved 4.2-inch model’s appearance by 1924, under the tutelage of Captain Lewis McBride. Like many of the weapons destined to play a part in World War II, the M2 suffered from the curtailed defense budgets of the 1920s and 1930s. Only a few of the mortars were produced, and they remained largely experimental with trials continuing essentially up to the war’s beginning.

One set of experiments beginning in 1934 involved creating a high-explosive round despite the M2 being earmarked as a chemical weapon delivery system. While no one at the time wanted to give up the M2’s primary role of launching gas and smoke, it would prove a fortuitous decision, making it a true dual-purpose weapon when war arrived years later.

Despite this extensive testing and planning, the four-deuce almost did not survive to see combat. In 1935, the War Department stopped all production of the weapon and later designated

the smaller 81mm mortar as standard issue for chemical battalions. The 81mm was an excellent light weapon for the infantry, but for the chemical branch it was considered much less capable. It was not until war clouds were looming close that the 4.2-inch was resurrected after a meeting between General George C. Marshall, Army chief of staff, and the commander of the Chemical Branch.

The M2’s potential as a close support weapon was obvious, and the chemical branch immediately began campaigning to adopt the high-explosive ammunition; while smoke munitions were of obvious use, the war thus far had seen no use of poison gas. It would be a waste of resources to relegate the mortar to screening missions only, and soldiers in the Chemical Warfare Service (CWS) saw it as a way to increase the role their branch would play in the war.

This would not be the last bureaucratic stumbling block for the M2. When the CWS presented the case for the mortar to the Army ground forces, it recommended the Field Artillery Board test the weapon first. Should the artillery approve the M2, it would be used as a substitute for the 105mm howitzer in theaters where that weapon was not in use.

Since there were few places the 105 did not go, it would be the death knell for the mortar. The services of Supply Branch supported the CWS, citing the mortar’s higher firepower, lighter weight, and lower manpower requirements for crews as reasons to adopt it alongside cannon. In the end, opposition simply died out, and high-explosive ammunition for the M2 was approved on March 19, 1943. The four-deuce was going to war.

The mortar that had been the center of such controversy had a caliber of 4.2 inches or 107mm. It was a conventional design in that it was muzzle loaded; the crew dropped rounds into the tube from the muzzle. The bomb would slide down the barrel, and its end would strike a firing pin that would detonate the propellant, sending the round back up the tube and on its way.

Unlike most mortars, the M2 had a rifled barrel to impart a spin to its projectile, increasing accuracy. The weapon was composed of three parts—the 175 pound baseplate that sat on the ground and absorbed the recoil of firing, a barrel 40.1 inches long, and the standard. This piece is connected to the baseplate by two connecting rods and has a single vertical piece that houses the elevating mechanism. The mortar is also equipped with an optical sight that lines up on aiming stakes similar to artillery. The entire assembly weighs 330 pounds.

The weapon could traverse seven degrees to the left or right, and elevation was between 45 and 60 degrees. Up to 20 rounds per minute could be fired for short periods, though this was often exceeded in combat. Minimum range was 600 yards. Maximum range, initially just over 2,000 yards, was improved by war’s end to 4,500 yards, again commonly exceeded in action. High-explosive bombs weighed in at about 25 pounds, while smoke rounds were heavier at 32 pounds. In practice the weapons were often called “chemical mortars” since they were only used by the CWS.

 

The early chemical mortar units varied in size from company to regiment. Standardization was soon introduced with the battalion as the standard unit, though a few separate companies did see service in the Pacific theater. Battalions had four companies, each with two platoons. A platoon was further divided into two sections, each with three mortars, totaling 12 weapons per company and 48 per battalion. By mid-1942, six battalions existed: the 2nd, 3rd, and 81st through 84th. Four more were created by mid-1943, the 85th through 88th.

Combat Debut in Italy

The baptism of fire for the M2 came with the invasion of Sicily, Operation Husky. Four battalions—2nd, 3rd, 83rd, and 84th—were chosen to go ashore. There were two weaknesses. Only two companies of the 3rd Battalion had received amphibious training, and only the 2nd had conducted training alongside infantry units. Time limitations before the landing meant little work could be done to correct the problems.

Nevertheless, the mortar units went ashore and quickly proved their worth. When the Rangers fought at Gela, the 83rd supported them, helping repulse enemy tank and infantry attacks. The 2nd Battalion finished off a disabled German armored vehicle that was firing on American troops, firing eight rounds that all landed in an area only 15 yards in diameter with one round going right into the open top of the vehicle. The mortarmen also laid large smokescreens to screen friendly movements, in one case keeping the screen up for nearly 14 hours. Unit commanders were generally very positive about the effectiveness of the chemical mortar support, smoke, and high explosive, one even calling it “the most effective single weapon used in support of infantry.”

Still, as with any weapon new to combat use, some shortcomings were noted. Users wanted a longer range. At this point the M5A1 propellant gave a maximum range of 3,200 yards (the 4,500-yard M6 propellant was not yet available). The sight was also criticized as having too small a traverse and lacking illumination for night firing. Using typical G.I. ingenuity, Americans adapted captured Italian 81mm mortar sights, which proved to be a better piece of equipment.

After Sicily, chemical mortar battalions went on to serve at SalernoAnzio, and throughout the Italian campaign. One battalion, the 83rd, became known as the “Artillery of the Rangers,” beginning its relationship with Colonel William Darby’s Ranger force at Gela on Sicily. The Rangers had no organic heavy weapons, and the M2’s firepower was a welcome addition. During the first two weeks of the Salerno campaign, C and D Companies alone fired more than 14,000 rounds. When the Rangers seized Chuinzi Pass and Mount Saint Angelo, they faced heavy counterattacks from German forces. The fighting became so desperate that mortarmen had to take their place on the line as infantry. Many targets were engaged up to 2,000 yards beyond the four-deuce’s range. The strain of such hard usage caused large numbers of breakages in mortar parts.